It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you recite 'Al-hamdu lillah' (All praise is for Allah), then also recite 'Bismillah al-Rahman al-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful). It is Umm al-Qur'an (the Mother of the Qur'an), Umm al-Kitab (the Mother of the Book) and al-Sab' al-Mathani (the seven oft-repeated verses), and 'Bismillah al-Rahman al-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful) is one of its verses."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2390